1. Field of the Invention
The present invention relates to a method of fabricating a thermooptic tunable wavelength filter which controls transmission wavelength according to thermooptic effect using a polymer optical waveguide and polymer Bragg grating.
2. Discussion of Related Art
A wavelength tunable filter using polymer Bragg grating has an advantage in very narrow bandwidth of transmission wavelength and its tunable wavelength characteristic using thermooptic effect produces stable operation characteristics. While optical fiber wavelength filter is widely known as a device using Bragg grating, the wavelength filter constructed in a manner that a polymer Bragg grating is integrated in a polymer optical waveguide is hardly known. The present invention firstly proposes a tunable wavelength filter which adjusts the effective refractive index of polymer optical waveguide in which the polymer Bragg grating is integrated using thermooptic effect, to control its transmission wavelength.
WDM has received considerable attention recently, because it provides an efficient way to increase the transmission capacity of optical communication systems. While previous optical communication systems transmitted optical signals using only one light source, WDM optical communication method uses multiple light sources each of which has different wavelengths to allocate each of various pieces of information to each of the different wavelengths, transmitting them. This increases the transmission capacity in linearly proportion to the number of wavelengths, reaching the level where information is transmitted in 10.sup.12 bit per second using only current techniques.
There is a method of precisely controlling the angle of bandpass filter using piezo-electric actuator to extract an optical signal having a specific wavelength, disclosed in U.S. Pat. No. 5,506,920, "Optical wavelength tunable filter" by Suemura, Yoshihiko, Japan, NEC corporation. This method provides stable operation of filter but has difficulty in realization of the filter small in size. There has been also proposed an optical filter, described in C. Kostrzewa et al., Technishe Universitat Berlin, Germany, "Tunable polymer optical add/drop filter for multiwavelength networks" in which unbalanced Mach-Zehnder interferometer is fabricated in cascade in a polymer optical waveguide to provide a tunable wavelength filter using the polymer optical waveguide, and interference characteristic is controlled using thermooptic effect to tune wavelength, providing relatively wide tunable ranges. However, it has a disadvantage in actual application due to wide band width of transmission signal.
In order to selectively extract only one wavelength desired in WDM optical communication system, it requires a tunable wavelength filter in which the band width is narrow and wavelength tuning range is sufficiently wide [M. S. Borella, J. P. Jue, D. Banerjee, B. Ramanurphy, B. Mukherlee, Proceeding of the IEEE, 85, 1274 (1997)]. The wavelength filter is needed to transmit only specific WDM optical signal and the crosstalk caused due to transmission of unwanted wavelength which is not required must be low less than -20 dB.
The wavelength filter fabricated in a manner that the Bragg grating is formed in an optical fiber has excellent characteristics, and is commercially available due to simplicity in its fabrication. However, the optical fiber Bragg grating is a passive device which cannot tune the transmission wavelength. A currently commercially available tunable wavelength filter is Fabry-Perot type device using two high reflection coated optical fibers. This can accurately mechanically adjust the distance between the optical fibers to control the wavelength transmitted through Fabry-Perot. However, the optical Fabry-Perot tunable wavelength filter has poor applicability because it is not integrated optical device.
Integrated optical waveguide tunable wavelength filters have been studied using various materials. Semiconductor optical waveguide structures include a grating-assisted codirectional coupler type filter described in Appl. Phys. Lett., 60, 980 (1992) by R. C. Alferness, L. L Buhl, U. Kohen, B. I Miller, M. G. Young, T. L. Koch, C. A. Burrus and G. Raybon, and multi-period Bragg reflector described in J. Lightwave Technol., 14, 2719 (1996) by J. P. Weber, B. Stoltz, H. Sano, M. Dasler, O. Ober and J. Walz. There has been proposed a nonbalanced Mach-Zehnder interferometer device using lithium niobate or polymer optical waveguide described in J. Lightwave Technol., 14, 2530 (1996) by E. L. Wooten, R. L. Stone, E. Miles and E. M. Bradley, and in IEEE Photon. Technol. Lett., 9, 1487 (1997) by C. Kostrzewa, R. Moosburger, G. Fischbeck, B. Schiippert, K. Pertermann. However, these devices cannot be commercially available yet due to their characteristic problems.